This application claims the priority of German Patent document 100 07 242.9 filed Feb. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for protecting the catalytic converter in the exhaust system of an internal combustion engine from overheating in the case of retarded ignition angles.
A catalytic converter protection apparatus known from German Patent DE 36 27 437 C2 detects overheating of the catalytic converter indirectly through the evaluation of the output signal of the lambda sensor. If overheating is detected, this is merely indicated, and no measures are taken to counteract the overheating.
In a control device described in German Reference DE 43 34 865 A1, the ignition angle retardation specified in the warm-up phase is stopped or at least reduced in the event of potential overheating, i.e. the exhaust-gas temperature is reduced by advancing the ignition angle during the warm-up phase. The manner in which this is achieved is not described. Monitoring and avoidance of overheating of the catalytic converter in normal operation are not provided.
The object of the present invention is to prevent the thermal destruction of the catalytic converter that occurs with excessively retarded ignition angles in an effective manner in all operating states while as far as possible maintaining the safety and comfort characteristics without restriction.
In the case of excessively retarded ignition angles, there is the risk of overheating and destruction of the catalytic converter. To avoid this, a parameter-dependent characteristic map is advantageously achieved by defining, in the respective operating state, an ignition angle that acts as a limiting ignition angle and allows more advanced ignition angles for any desired period of time. Only when a calculated optimum ignition angle exceeds this limiting ignition angle in the retarded direction is there a risk of overheating. To make the loss of comfort that must be accepted as small as possible and maintain a control response of vehicle-dynamics programs and transmission programs as optimum as possible, the respectively optimum angle and, in a subsequent recovery phase, the unrestrictedly specifiable limiting ignition angle, are advantageously specified in alternation by a timing device. This is an effective means of stopping or limiting the heating of the catalytic converter, making it possible to minimize the effects on driving operation.
The timing is expediently engine-speed-dependent, i.e. the time periods are specified at least as a function of the engine speed, allowing them to be optimized with regard to the respective operating state with a view to preventing overheating.
In an advantageous refinement, the time periods for the alternating switch between the two ignition angles is specified by means of a ramp generator using a ramp function, preferably through up/down ramps.
However, because excessively retarded ignition angles could lead to overheating, the optimum ignition angle is advantageously limited by means of a limiting device, using an ignition angle retarded to the maximum extent. In this context, the ignition angle retarded to the maximum extent is preferably specified by a characteristic map or a characteristic at least as a function of the engine speed.
Since the risk that the catalytic converter will overheat is significantly lower in the warm-up phase, the unrestrictedly specifiable ignition angle and/or the ignition angle retarded to the maximum extent can then be retarded by a warm-up ignition angle, allowing more retarded optimum ignition angles in this phase, leading to enhanced comfort and better quality of control, especially in the warm-up phase.
During certain specifiable dynamic processes, the cyclic change to the ignition angle, which can be specified without time restriction, can be switched off, i.e. in such special situations the optimum ignition angle is then specified even if this is more retarded than the unrestrictedly specifiable ignition angle. Such brief processes do not lead to overheating of the catalytic converter and it is therefore possible to accept more retarded ignition angles in these situations with a view to better dynamic processes.
If icing is detected at the throttle valve and/or at other locations, the system switches to a correspondingly optimized ignition-angle characteristic map, which is then used for ignition of the internal combustion engine. Thus, there is also a departure from the specification of the ignition angle in accordance with the invention in such special situations.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.